Laminated fuel tank



Nov. 18, 1947.

A. HERSHBERGER LAMINATED FUEL TANK Filed Aug. 18, 1943 Paiemed Nov. is, 1947 LAMINATED FUEL TANK Albert Hershberger, Kenmore, N. Y., assignor to v E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation oi' Delaware Application August 18, 1943, Serial No. 499,109

'I'his invention relates to tanks or containers for the shipment or storage of organic liquids such as motor fuels, especially those motor fuels that contain aromatic hydrocarbons. More particularly, the invention relates to a laminated wall structure for self-sealing fuel tanks comprising a novel barrier member which serves to render the tank insensitive and impermeable to the organic liquids and their vapors.

Because of their exceptional resistance to the deteriorating influence of gasoline and relatively low penetrability by gasoline vapors, compounded stocks of rubber-like vulcanizable butadiene copolymers such as the copolymers of butadiene with acrylonitrile have been widely employed in the construction of self-sealing fuel tanks for military planes and similar motorized equipment. However, with the advent oi' gasoline of very high octane rating due to the presence therein of aromatic hydrocarbons, the impenetrability of these copolymers is no longer entirely satisfactory and the need arises for materials exhibiting still higher resistance to penetration by these fuels.

It is known that many self-sustaining, nonvulcanizable, non-fibrous, synthetic films possess the desired impenetrability to such fuels and it has been proposed to laminate these lms to the above copolymers of'butadiene to form a structure suitable for use in the manufacture of these tanks. These laminated structures have generally proven unsatisfactory, however, because although these synthetic films possess the desired impenetrabllity to the motor fuel, they do not possess in addition both the required flexibility and durability at low temperatures and the necessary resistance to water andmoisture vapor. These tanks, particularly when installed in airplanes, are often subjected to very low temperatures. To be satisfactory for use, they must of course retain their flexibility 'and resist shattering under impact under these conditions. At the same time they must resist the action of water, whether formed by the condensation of'moisture vapor at these low temperatures or present during the manufacture of the tank, for example, as steam during the vulcanizing step.

It is therefore an object of this invention to provide a laminated wall structure for fuel tanks, which structure comprises a fuel barrier which is substantially impervious to high octane motor fuels containing aromatic hydrocarbons. Another object is to provide in a laminated wall structure for self-sealingl fuel tanks a fuel barrier of self-sustaining, non-vulcanizable, nonfibrous, synthetic film which is substantially im- 1 Claim. (Cl. 154-435) pervious to high octane motor fuels containing aromatic hydrocarbons. which is flexible and durable at low temperatures, and which is resistant to water and moisture, l, e. does not tend to swell or dissolve in water.

These and other objects will more clearly appear in the following description.

I` have discovered that films prepared from certain partial acetals o1' polyvinyl alcohol, name 1y those wherein from 12 to 55% of the available hydroxyl groups of the polyvinyl alcohol are combined with an aliphatic aldehyde containing from one to six carbon atoms, are not only resistant to the action of and impermeable to the vapors of motor fuels including those that contain appreciable quantities of aromatic hydrocarbons, but are also resistant to water or moisture vapor and possess a satisfactorily high degree of flexibility, and durability at low temperatures such as temperatures of 30 C.

The objects of this invention are therefore accomplished by the use of a lm of a partial acetal of polyvinyl alcohol (wherein from 12 to ofthe available hydroxyl groups are combined with .an aldehyde containing from one to six carbon atoms) in forming a, laminated structure for use in motor fuel tanks, particularly fuel tanks of the self-sealing types.

'I'he partial polyvinyl acetals suitable for purposes of this invention are preferably prepared by the treatment of an aqueous solution of polyvinyl alcohol with an aldehyde containing from one to six carbon atoms in the presence of a suitable catalyst such as sulfuric acid, organic sulfonic acids, etc. Thereactionfconditions, including concentration ofthe reaction mixture, amount of catalyst employed, time and temperature of the reaction, etc., are so controlled as to yield a partial acetal of the desired composition. The proper conditions for the preparation of any given acetal can, of course, be determined by analysis of the final product.

When the aldehyde employed in the preparation of the partial acetal is butyraldehyde, the reaction conditions are preferably so controlled that from 35 to 55% of the available hydroxyl groups of the polyvinyl alcohol react with the aldehyde (butyral content of from 23.7 to 33.6%). When the aldehyde employed is of lower molecular weight than butyraldehyde, e. g. formaldehyde, acetaldehyde or propionaldehyde, the reaction conditions are so chosen that from 45 to .of the available hydroxyl groups of polyvinyl alcohol react. When the aldehyde employed contains ve or six carbon atoms, the reaction conditions are preferably so adjusted that from 12 to 40% of the available hydroxyl groups of the polyvinyl alcohol react. The resulting partial acetals are soluble in mixtures of alcohol and water, or acetone and water, and may be cast in known manner from such solution to form films for use in the fuel tanks of this invention. AIn general, it, is preferred that these films be approximately 0.002 thick, although this is not critical to the invention. Suitable methods for forming the film are described in U. S. Patents Nos. 2,044,730, 2,045,130, 2,120,628 and- 2,254,078.

The following example illustrates the preparation of'a partial polyvinyl butyral film suitable for use with this invention. Parts are by weight.

Example The temperature fis then raised to 30 C. and the solution stirred for 30 minutes after which time the temperature is raised to 40 C. f-or another period of 30 minutes. The solution is then heated to 65 C. and 0.88 part of sulfuric acid dissolved in 100 parts of water is added, stirring being continued at this temperature for an additional period of 30 minutes. The solution is then neutralized by the addition of aqueous sodium hydroxide and after stirring for an additional period of 30 miny utes, the precipitated polyvinyl butyral is removed by filtration, washed with water and dried. Analysis shows that the material has a butyral content of 30.3, corresponding to a degree of substitution of 48% of the available hydroxyl groups of the original polyvinyl alcohol.

The dried product is dissolved in a mixture of 80 parts methanol and 20 parts water to form a 15% solution, 0.5% dilauryl phosphate and 5% glycerol (on the polyvinyl butyral basis) being added to the solution. The solution was cast onto a heated surface to form a iilm 0.002 thick. The resulting film, after evaporation of the solvent, was glass-clear and substantially unaffected by immersion in water, and was substantially impervious to a motor fuel comprising 60 parts of 100 octane gasoline, 5 parts benzene, 20 parts toluene, and 15 parts xylene. The film was bonded to "Hy'car (a compounded acrylonitrilebutadiene copolymer; Ind. & Chem., May 1941, pp. 602-6) by an adhesive of the type disclosed in my copending application Serial No. 459,187, filed September 21, 1942 (U. S. P. 2,411,878). When this laminated structure was cooled to 30 C. and fired at with a 45-caliber revolver, the bullet entering the Hycar side ofV the lamination, the film did not shatter.

The application of the polyvinyl acetal film according to the principles of my invention will now be described in connection with the accompanying drawing wherein:

Fig. 1 is a view, partly in cross-section, of a typical fuel tank provided with a preferred form of laminated wall construction;

'Figs 2, 3 and 4 illustrate in cross-sectional view modified wall structures embodying my invention.

Referring to Fig. 1. the inner layer of the 4 laminated wall structure (that layer in contact with the motor fuel) comprises a film 10 of the partial polyvinyl acetal of this invention, the film being approximately 0.002 thick. On its outer side the lm is bonded to a layer 11 of a compounded rubber -like vulcanizable butadiene copolymer, preferably the copolymer of butadiene with acrylonitrile. Any satisfactory adhesive can be employed to bond the film of this invention to the copolymer. However, it is preferable to use an adhesive of the type disclosed in copending application Serial No.459,187, filed September 21, 1942, comprising a polyvinyl butyral having a butyral content of 20.7% to 47.4% and a polyhydric phenolaldehyde resin. Successive layers of the laminated structure comprise a fabric I3 coated on the one side with the copolymer of layer /II and on theother side with mildly cured gum rubber, a layer 14 of natural gum rubber, and fabric I5 and I6 into which gum rubber has been calendered. The entire structure is, of course, cured by means of steam or heated air to form it into a single laminated structure. Because of the presence of the inner layer l0 of the partial polyvinyl acetal of this invention, the structure resists the action of and is highly impermeable to motor fuels including those fuels having a high octane number due to the presence therein of aromatic hydrocarbons. Because of the presence of layer I4 of natural gum rubber, the structure is rendered self-seal ing in nature when penetrated by a bullet or other projectile. This effect is, of course, due to the swelling of the natural gum rubber by the motor f fuel coming in contact with it.

The exact composition of the laminated structure of the fuel tank can, of course, be varied greatly without departing from the principles of this invention so long as at least one layer 0f the laminated structure comprises the polyvinyl partial acetal of this invention. For example, as shown in Fig. 2, the layer Il of butadiene copolymer can be omitted from the structure, the film I0 of polyvinyl partial acetal being bonded directly to the calendered fabric I3 0f Fig. 1. Such a structure will, of course, still resist the action of and be impermeable to motor fuels. Moreover, because it contains a layer of mildly cured gum rubber, the structure will still be selfsealing in nature. On the other hand, the positioning of an additional reinforcing layer 9 on the inner side of the film I0 of Fig. 1 (in the manner of Fig. 3) aids in strengthening the final tank structure. Such construction is contemplated by the invention. This reinforcing layer 9 can, if-desired, comprise a film of synthetic rubber such as that indicated by the reference number lll of Fig. 1.

Of course, if it is not necessary that the tank be so strongly reinforced or that it be self-sealing in nature, the structure can be greatly simplifled. In such a case, as shown in Fig. 4, the structure can comprise merely a film I0 of the polyvinyl acetal of this invention bonded to one or more reinforcing fabrics I3 and I4. If the fabric I3 has been impregnated or calendered with a copolymer of butadiene and acrylonitrile, the partial acetal film can be bonded to the fabric by the adhesive disclosed in my above-mentioned copending application. If, 0n the other hand, the supporting fabric has been impregnated with a polyvinyl partial acetal of the type disclosed in this invention, a satisfactory bond between the acetal film and the fabric can be obtained by merely moistening the film or fabric with a solvent for the acetal and pressing the two members together.

Nor is it necessary that the 111m I0 of this invention be preformed in a separate operation be- Of course, several successive coating operations may be used to insure this. In those cases where an adhesive is required to bond the film oi this invention tothe adjacent member, that member must, of course, be provided 'with the adhesive prior to the coating operation.

This invention, which is based on the discovery that lms of certain polyvinyl partial aeetals are at one and the same time water-resistant, highly flexible and durable, particularly at low temperatures, and. resistant to the action of and impermeable to the vapors of motor fuels containing aromatic hydrocarbons, makes possible the construction of eminently satisfactory tanks for use in the storage of motor fuels, including fuels of high octane ratingydue to the presence of aromatic hydrocarbons. Because these films of polyvinyl partial acetals can be readily and rmly bonded (either directly or through an intermediate layer, such as a layer of a butadiene copolymer) to materials suchas natural gum rubber which are highly swollen on contact with these motor fuels, theymake possible the construction of self-sealing tanks for use with these fuels.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it

-is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claim.

I claim:

A container adapted for the storage or shipment of high octane motor fuels and the like comprised of laminated walls comprising as essential laminae, a nlm of a partial acetal of` a polyvinyl alcohol in which from 12% to 55% of the available hydroxyl groups of the polyvinyl alcohol are combined with an aliphatic aldehyde containing from one to six carbon atoms, a layer of a compounded rubber-like vulcanizable 20 butadiene copolymer, and a layer of natural gum rubber.

ALBERT HERSHBERGER.

REFERENCES CITED The following references are of record in the nie of this patent:

UNITED STATES PATENTS so Number Name Date 2,160,371 Schnabel May 30, 1939 2,053,112 Schnabel Sept. 1, 1936 2,250,674:V McBurney July 29, 1941 1,297,305 Thacher Mar. 11, 1919 35 1,779,397 Kraft Oct. 21, 1930 2,120,628 Reid June 14, 1938 

